The rolling bearing unit is used to support rotatably the wheel of the vehicle with the suspension system. Also, the rotational speed of the wheel must be sensed to control various vehicle attitude stabilizing system such as the anti-lock brake system (ABS), the traction control system (TCS), and so on. As a result, recently not only to support rotatably the wheel with the suspension system but also to sense the rotational speed of this wheel is widely carried out by the rolling bearing unit equipped with the rotational speed detection device in which the rotational speed detection device is incorporated into the rolling bearing unit.
As the rolling bearing unit equipped with the rotational speed detection device used for such purpose, a number of structures such as the structure set forth in JP-A-2001-21577, etc. are known. The ABS or the TCS can be controlled appropriately by feeding a signal indicating the rotational speed of the wheel, which is sensed by the rolling bearing unit equipped with the rotational speed detection device, to the controller. In this manner, the stability of the running attitude of the vehicle at the time of braking or acceleration can be assured by the rolling bearing unit equipped with the rotational speed detection device, nevertheless the brake and the engine must be controlled based on full information, which have an influence on the running stability of the vehicle, to assure this stability under more severe conditions. In contrast, in the case of ABS or TCS utilizing the rolling bearing unit equipped with the rotational speed detection device, the brake and the engine are controlled by sensing the slip between the tire and the road surface, i.e., so-called feedback control is executed. For this reason, since the control of the brake and the engine is delayed of course, though such delay is only an instant, improvement in such control is desired from an aspect of the performance improvement under the severe conditions. Namely, in the case of the related-art structure, so-called feedforward control can prevent neither. generation of the slip between the tire and the road surface nor so-called one-sided activation of the brake, i.e., the event that braking powers are extremely different between left and right wheels. In addition, such control cannot prevent the event that the running stability of the truck, or the like becomes worse due to its improper carrying state.
In light of such circumstances, the rolling bearing unit equipped with the load measuring device shown in FIG. 37 is disclosed in JP-A-2001-21577. In this rolling bearing unit equipped with the load measuring device in the related art, a hub 2 is fitted to the inner diameter side of an outer ring 1. Such hub 2 couples/fixes the wheel and acts as a rotating ring and also an inner-ring equivalent member. Such outer ring 1 is supported with the suspension system and acts as a stationary ring and also an outer-ring equivalent member. This hub 2 includes a hub main body 4 having a rotation side flange 3 at its outer end portion (end portion positioned on the out side in a width direction in a fitted state to the vehicle) to fix the wheel, and an inner ring 6 fitted to an inner end portion (end portion positioned on the center side in the width direction in the fitted state to the vehicle) of the hub main body 4 and fixed with a nut 5. Then, a plurality of rolling elements 9a, 9b are aligned respectively between double row outer ring raceways 7, 7 and double row inner ring raceways 8, 8. Such double row outer ring raceways 7, 7 are formed on an inner peripheral surface of the outer ring 1 to act as a stationary side raceway respectively. Such double row inner ring raceways 8, 8 are formed on an outer peripheral surface of the hub 2 to act as a rotation side raceway respectively, such that the hub 2 can be rotated on the inner diameter side of the outer ring 1.
A fitting hole 10 for passing through the outer ring 1 in the diameter direction is formed in a middle portion of the outer ring 1 in the axial direction between the double row outer ring raceways 7, 7 and in an upper end portion of the outer ring 1 in the almost perpendicular direction. Then, a round lever (rod-like) displacement sensor 11 serving as a load measuring sensor is fitted into the fitting hole 10. The displacement sensor 11 is of non-contact type, and a sensing face provided to its top end surface (lower end surface) is opposed closely to an outer peripheral surface of a sensor ring 12 that is fitted to the middle portion of the hub 2 in the axial direction. When a distance between the sensing face and the outer peripheral surface of the sensor ring 12 is changed, the displacement sensor 11 outputs a signal in response to an amount of change in the distance.
In the case of the rolling bearing unit equipped with the load measuring device constructed as above in the related art, the load applied to the rolling bearing unit can be measured based on a sensed signal of the displacement sensor 11. In other words, the outer ring 1 supported with the suspension system of the vehicle is pushed down by the weight of the vehicle whereas the hub 2 for supporting/fixing the wheel still acts to stay at that position as it is. Therefore, a deviation between a center of the outer ring 1 and a center of the hub 2 is increased based on elastic deformations of the outer ring 1, the hub 2, and the rolling elements 9a, 9b as the weight is increased more and more. Then, a distance between a sensing face of the displacement sensor 11, which is provided to the upper end portion of the outer ring 1, and an outer peripheral surface of the sensor ring 12 is reduced as the weight is increased more and more. Accordingly, if the sensed signal of the displacement sensor 11 is fed to the controller, the load applied to the rolling bearing unit which is equipped with the displacement sensor 11 can be calculated based on a relational expression derived by the experiment or the like previously, a map, or the like. Based on the loads applied to the rolling bearing units and sensed in this manner, the ABS can be controlled properly and also the driver is informed of the improper carrying state.
In this case, the related-art structure shown in FIG. 37 can sense a rotational speed of the hub 2 in addition to the radial load applied to the rolling bearing unit. For this purpose, a rotational speed encoder 13 is fitted/fixed to the inner end portion of the inner ring 6 and also a rotational speed sensor 15 is secured to a cover 14 that is put on an inner end opening portion of the outer ring 1. Then, a sensing portion of the rotational speed sensor 15 is opposed to a sensed portion of the rotational speed encoder 13 via a sensing clearance.
In operation of the rolling bearing unit that is equipped with the above rotational speed detection device, an output of the rotational speed sensor 15 is changed when the rotational speed encoder 13 is revolved together with the hub 2, to which the wheel is fixed, and then the sensed portion of such rotational speed encoder 13 passes through in vicinity of the sensed portion of the rotational speed sensor 15. In this way, a frequency of an output of the rotational speed sensor 15 is in proportion to the number of revolution of the wheel. Therefore, if the output signal of the rotational speed sensor 15 is supplied to the controller (not shown) provided to the vehicle body side, the ABS or the TCS can be controlled appropriately.
The related-art structure set forth in above JP-A-2001-21577 measures the radial load applied to the rolling bearing unit whereas, in JP-A-3-209016, the structure for measuring a magnitude of the axial load applied to the rolling bearing unit via the wheel is set forth. In the case of the related-art structure set forth in JP-A-3-209016, as shown in FIG. 38, the rotation side flange 3 used to support the wheel is fixed to an outer peripheral surface of an outer end portion of a hub 2a that acts as the rotating ring and the inner ring equivalent member. Also, double row inner ring raceways 8, 8 that correspond to a rotation side raceway respectively are formed on an outer peripheral surface of the middle portion or the inner end portion of the hub 2a. 
Meanwhile, a stationary side flange 17 to support/fix the outer ring 1 to a knuckle 16 constituting the suspension system is fixed to an outer peripheral surface of the outer ring 1, which is arranged around the hub 2a in a concentric manner with this hub 2a and acts as the stationary ring and the outer ring equivalent member. Also, the double row outer ring raceways 7, 7 that correspond to a stationary side raceway respectively are formed on the inner peripheral surface of the outer ring 1. Then, a plurality of rolling elements (balls) 9a, 9b are provided rotatably between the outer ring raceways 7, 7 and the inner ring raceways 8, 8 respectively, whereby the hub 2a is supported rotatably on the inner diameter side of the outer ring 1.
In addition, a load sensor 20 is affixed to portions that surround screwed holes 19, into which a bolt 18 is screwed respectively to couple the stationary side flange 17 with the knuckle 16, at plural locations on the inner side surface of the stationary side flange 17 respectively. In a state that the outer ring 1 is supported/fixed to the knuckle 16, these load sensors 20 are held between the outer surface of the knuckle 16 and the inner surface of the stationary side flange 17.
In the case of such load measuring device for the rolling bearing unit known in the related art, when the axial load is applied between the wheel (not shown) and the knuckle 16, the outer surface of the knuckle 16 and the inner surface of the stationary side flange 17 are pressed against respective load sensors 20 from both surfaces in the axial direction. Therefore, the axial load applied between the wheel and the knuckle 16 can be sensed by summing up measured values of these load sensors 20. Also, in JP-B-62-3365 that, although not shown, the method of calculating the revolution speed of the rolling elements based on a vibration frequency of the outer ring equivalent member, a part of which has a low rigidity, and then measuring the axial load applied to the rolling bearing unit is set forth.
Out of the structures that measure the load (the radial load or the axial load) applied to the rolling bearing, as described above, in the case of the first example of the related-art structure shown in above FIG. 37, the load applied to the rolling bearing unit is measured by measuring respective displacements of the outer ring 1 and the hub 2 in the radial direction by means of the displacement sensor 11. In this case, because an amount of displacement in the radial direction is minute, a high-precision sensor must be used as the displacement sensor 11 to measure the load with good precision. Since a high-precision non-contact type sensor is expensive, it is inevitable that a cost is increased as the overall rolling bearing unit equipped with the load measuring device.
Also, in the case of the structure for measuring the axial load as the second example of the related-art structures shown in FIG. 38, the load sensors 20 must be provided to the knuckle 16 as many as the bolts 18 used to support/fix the outer ring 1. For this reason, in addition to the fact that the load sensor 20 itself is expensive, it is inevitable that a cost of the overall load measuring device for the rolling bearing unit is considerably increased. Also, in the method set forth in JP-B-62-3365, the rigidity of the outer ring equivalent member must be reduced partially and thus there is such a possibility that it is difficult to assure the endurance of the outer ring equivalent member.